Ultrawideband (UWB) phased arrays are desirable for use in high-throughput wireless communication systems, such as cellular and satellite systems, as well as radar, electromagnetic countermeasure, and multifunctional (communications/sensing) systems. Currently, the dominant UWB array technologies require elaborate vertical integration, are non-planar and often require 3D machined parts (feed organizer) along with external baluns or hybrid circuits. Vertical integration, 3D machining and non-modular assembly are particularly problematic in phased array technologies because a large number (100-7000) of elements must be integrated together, leading to very high recurring costs. In addition, these arrays face challenges when conformal mounting is required. Also, fabrication at millimeter-wave frequencies is intractable because the required manufacturing and integration technologies do not scale to smaller sizes without significant cost penalties. For that reason these arrays are prohibitive for commercial applications (which require very low recurrent fabrication costs) and are typically used at the lower frequency bands (L, C, X bands) for defense applications. A fully planar, modular UWB array that can be scaled to higher frequencies could have significant impact on current and future commercial as well as defense systems.
Microstrip patch arrays, while fully planar and easy to fabricate, offer limited bandwidths. A typical microstrip antenna in isolation, fed by a microstrip line or a probe, has less than 5% fractional bandwidth, while fractional bandwidth up to 50% have been reported using aperture feeding, stacked patches, thick substrates, L-shaped feeds, or other broadbanding techniques. When used in arrays, designs have achieved moderate bandwidths, such as Edimo, who reported a 16% fractional bandwidth using an array of aperture fed stacked patches (M. Edimo, P. Rigoland, and C. Tenet, “Wideband dual polarized aperture coupled stacked patch antenna array operating in C-band”, Electronics Letters, IEEE, vol. 30, pp. 1196-1197, July 1994.), while Lau has reported 20% fractional bandwidth using L-probe fed stacked patches (Lau, K. L.; Luk, K. M., “A Wideband Dual-Polarized L-Probe Stacked Patch Antenna Array,” Antennas and Wireless Propagation Letters, IEEE, vol. 6, pp. 529-532, 2007.). While these bandwidths are high compared to that of a typical microstrip patch antenna, these planar apertures do not offer large enough bandwidths for multifunctional UWB applications.
A second quasi-planar technology that can offer moderate bandwidths are dielectric resonator arrays (DRA), which are comprised of arbitrarily shaped 3D dielectric slabs attached to a substrate. These resonators are fed with microstrip lines, slots, or probes, similar to patch arrays. Although not fully planar, DRAs are simple to fabricate and have low profile. Arrays have been designed with bandwidths on the order of a few percent, such as an array presented by Oliver (“Broadband Circularly Polarized Dielectric Resonator Antenna” U.S. Pat. No. 5,940,036) which has a fractional bandwidth of 5%, while others have reported fractional bandwidths up to 21% (“Dielectric Resonator Antenna With Wide Bandwidth” U.S. Pat. No. 5,453,754). As with the microstrip patch arrays, DRAs offer simple fabrication and feeding, but do not offer the high bandwidths appropriate for UWB applications.
The first quasi-planar array that offers UWB operation is the Current Sheet Antenna (CSA). This array is based on Wheeler's current sheet concept (H. Wheeler, “Simple relations derived from a phased-array antenna made of an infinite current sheet,” IEEE Transactions on Antennas and Propagation, vol. 13, no. 4, pp. 506-514, July 1965). Ben Munk realized Wheeler's current sheet with a periodic array of closely packed horizontal dipoles, placed λ/4 above an infinite ground plane. The capacitance of the short dipoles is counteracted by the inductance of the ground plane, leading to large bandwidths. A practical implementation of this array concept was disclosed by R. Taylor and B. Munk (“Wideband Phased Array Antenna and Associated Methods”, U.S. Pat. No. 6,512,487 B1), which is comprised of periodically placed crossed dipoles with coincident-phase center feeds and with large interdigitated capacitors between neighboring dipoles. The elements are placed λ/4 (at midband) distance away from the ground plane, and, since dipoles are balanced structures, an external balun must be attached at each port to connect each element to standard (unbalanced) transmission lines. The array allows for single or dual polarization and has high efficiency, planar aperture layer, good scan performance, and a reported bandwidth up to 9:1 (160% fractional bandwidth). However, while the element layer with elements 106 and 107 is planar, the feed structure consists of a 3D metallic structure 127, see FIGS. 1B and C.
Feed structure 127 is called the “feed organizer”; two different style feed organizers have been developed for the CSA array (“Patch Dipole Array Antenna and Associated Methods”, U.S. Pat. No. 6,307,510, and “Patch Dipole Array Antenna Including A Feed Line Organizer Body And Related Methods”, U.S. Pat. No. 6,483,464 B2). The feed organizer isolates the four (assuming dual-pol) vertical balanced feed lines (e.g., lines 103 and 104), provides a ground reference (ground plane labeled 101), and suppresses a common mode that would otherwise develop if the feed lines were unshielded. The use of this elaborate feed device is critical for the CSA operation, since the development of common mode reduces the array bandwidth significantly. In addition to the complexity and cost of 3D metal feed organizers, the balanced feed lines require an external balun (128, shown in FIG. 1B) in order to interface with common unbalanced microwave transmission lines. This external balun in the feed network adds complexity and size to the feed network.
The CSA has been implemented in various additional forms. One implementation uses square patch elements densely arranged to achieve high capacitive coupling between elements and obtains a 2:1 bandwidth with scanning out to θ=45° and return loss <−10 dB (“Patch Dipole Array Antenna and Associated Methods”, U.S. Pat. No. 6,307,510). Another CSA design uses two stacked layers of CSAs operating at different bands to form large bandwidth arrays, such as those disclosed by Rawnick (U.S. Pat. No. 6,552,687 B1 and U.S. Pat. No. 6,771,221 B1), and by Croswell (U.S. Pat. No. 6,876,336 B1). Rawnick also disclosed a modular implementation that divided the array aperture along the gap between neighboring elements, forming tiles containing two orthogonal dipole elements (“Phased Array Antenna Formed As Coupled Dipole Array Segments” U.S. Pat. No. 7,463,210 B2). This arrangement removes the possibility of interdigitated capacitors between dipole elements; instead a set of metal plates are arranged across the boundary of neighboring tiles to achieve the required high capacitive coupling between the same polarization elements.
The second quasi-planar aperture topology capable of delivering UWB operation is the Fragmented Aperture Antenna (FAA), (“Fragmented aperture antennas and broadband antenna ground planes”, U.S. Pat. No. 6,323,809 B1 Maloney). The array is comprised of electrically connected, balanced metallic elements with complex shapes generated via numerical optimization techniques. To optimize performance, the element shapes are derived using discrete metal squares as building blocks, which are then arranged using genetic algorithms to optimize the bandwidth. As a result, the array achieves very wideband operation, with reported bandwidths up to 33:1 (fractional bandwidth of 188%) in dual or single polarized configurations, where the array has coincident phase center feeding when dual polarized. As with the CSA, the FAA requires a 3D metal feed organizer, external baluns and impedance transformers. A more serious drawback arises when unidirectional radiation is required from the FAA. When the array is backed by a ground plane, a series of catastrophic resonances appear in the band of operation. To remedy these resonances the FAA uses circuit analog absorbers or Jaumann screens. These structures are lossy and dramatically reduce the efficiency and power handling capability of the array, while in the receive mode they increase the antenna noise figure. A 1-2.8 dB reduction in gain is typical, indicating that in some cases nearly half of the input power is lost to heat in the resistive cards.
It is clear from the above discussion that only balanced (dipole-type) structures have thus far succeeded in offering UWB array operation. Since all balanced structures require an external balun or hybrids to connect to standard RF interfaces, the balun is a major component of the design. Over the years, much work has been done on integrated balun implementations for dipole elements. For example, U.S. Pat. No. 3,747,114 issued to Shyhalla shows a dipole array with baluns printed on the backplane, with the balun consisting of phase delay lines between the balanced feed pins of the dipole elements. Another example of an integrated balun is disclosed in U.S. Pat. No. 3,845,490 issued to Manwarren et al, which shows a stripline dipole structure fed by an “L” shaped transmission line embedded between the dipole layers. In U.S. Pat. No. 4,825,220, Edward et al demonstrates a “J” shaped microstrip line (also known as a Marchand balun) feeding a microstrip dipole structure that achieves 40% fractional bandwidth with VSWR <2. U.S. Pat. No. 5,892,486 issued to Cook et al also incorporated a “J” shaped microstrip line feeding a microstrip dipole where the “J” shaped balun extended above the dipoles. Pickles developed coincident phase center dipole arrays fed with double Marchand baluns that demonstrated a fractional bandwidth of 100% (W. R. Pickles and W. M. Dorsey, “Proposed Coincident Phase Center Orthogonal Dipoles,” Antenna Applications Symposium, pp. 106-124, 18-20, Sep. 2007. Monticello, Ill.). All of these solutions are relatively narrowband and require vertical integration (at least for the feeding section).
In the above discussion, it is clear that fully planar, unbalanced structures that can be directly fed by standard RF interfaces are narrowband, e.g. patch arrays and DRA. On the other hand, UWB arrays such as CSA or FAA are not fully planar (only the aperture layer is planar, with feed organizers or 3D machined parts that require non-planar integration and assembly) and require external baluns or hybrids. Any attempt to integrate baluns into planar arrays has yielded low bandwidth and must be vertically integrated. If low-cost, scalable UWB arrays are to be a reality, then a fully planar UWB array with integrated balun is necessary.